Thermal load reducing system for nuclear reactor vessel

ABSTRACT

In a reactor vessel thermal load reducing system near the surface level of a coolant, the present invention is characterized in that a heat conductive member is installed not contacting the reactor vessel ( 1 ) wall in an area above and below the coolant liquid surface, and the heat conducting member is a guard vessel ( 2 ) made of good heat conductivity, or a heat conductive plate ( 20 ) made of good heat conductivity.

This application is a divisional of application Ser. No. 10/682,859,filed Oct. 14, 2003, which is based upon and claims priority of JapanesePatent Application No. 2003-057147, filed on Apr. 3, 2003, the contentsbeing incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a thermal load reducing system for anuclear reactor vessel, available for the reduction of thermal load neara coolant liquid surface of a reactor vessel and for the reduction ofthermal load near a temperature stratified layer in a reactor vessel.

A reactor vessel in a fast breeder reactor is supported at its upper endby a concrete wall, which must be maintained at the temperature of 100°C. or lower. Because it has a high-temperature coolant at 550° C. orhigher in a plenum above reactor core, there occurs a steep temperaturegradient in the vertical direction from the coolant liquid surface tothe upper supported end. In particular, during starting operation, bothtemperature and liquid level rise at the same time, the gradient becomessteeper. As a result, high thermal stress develops, in principle, on thereactor wall near the liquid surface where the temperature gradientdeflects.

To cope with this problem, attempts have been made in the past toprevent the rise of liquid level using a liquid level controller, toevenly cool down the reactor wall using a reactor wall cooling system,and to reduce the bending stress by designing in a thin-wall structure.Also proposed is a reduction method of the temperature gradient near theliquid surface, arranging a liner to form a heat insulated space fromreactor vessel wall below the coolant liquid surface to directly belowthe seal plug, in cooperation with reactor vessel, and by filling heatinsulating material into the heat insulated space (Japanese PublicationNumber JP-A-57-80594).

As described above, the conventional method for reducing thermal loadhas its principal aims to prevent the rise of liquid level using aliquid level controller, to evenly cool down the reactor wall using areactor wall cooling system, and to decrease bending stress by designingin a thin-wall structure. The liquid level controller and the reactorwall cooling system result in higher cost because of the increase ofsystem components. For designing the system in a thin-wall structure,there was a limitation due to the possibility of other failure modes.The method described in the patent document referred to above, alsoleads to higher cost due to the increase of system components.

SUMMARY OF THE INVENTION

The object of the present invention is to provide sure operations and tocontribute, without giving a significant impact to the constructioncost, both for the increased safety of the reactors and for the improvedeconomy of the plant by reducing the thermal load itself, being thecause to generate stress.

For this sake, in a thermal load reducing system for reduction of stressnear a coolant surface of a reactor vessel, the present invention ischaracterized in that a heat conductive member is installed outside thereactor vessel in the area above and below the coolant liquid surfacenot contacting to the reactor vessel wall.

Further, the present invention is characterized in that aforementionedheat conductive member is a plate supported by a guard vessel.

Further still, the present invention is characterized in that theaforementioned heat conductive member is the guard vessel wall.

Further still, the present invention is characterized in that theaforementioned heat conductive member is of better material in heatconductivity than that of the reactor vessel.

Further still, the present invention is characterized in that theaforementioned good heat conductive material is high chrome steel.

Further still, the present invention is characterized in that theaforementioned good heat conductive material is graphite.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing to show an embodiment example of a thermal loadreducing system for reduction of stress near the liquid surface of areactor vessel;

FIG. 2 is a drawing to explain the principle of thermal load reductionnear a coolant liquid surface of a reactor vessel;

FIG. 3 is a drawing to show the relationship between equivalent heattransfer coefficient due to radiation and temperature in use;

FIG. 4 is a drawing to show an analysis mesh model;

FIG. 5 is a drawing to show an analysis mesh model zoomed in at the partof heat conductive plate;

FIG. 6 is a drawing to show the difference in generated stressesobtained as the result of the analysis between the cases with or withoutheat conductive plate;

FIG. 7 is a drawing to show another embodiment example of the thermalload reducing system for reduction of stress near the liquid surface ofthe reactor vessel;

FIG. 8 is a drawing to explain the principle of thermal load reductionnear the coolant liquid surface of the reactor vessel;

FIG. 9 is a drawing to show the relationship between equivalent heattransfer coefficient due to radiation and temperature in use;

FIG. 10 is a drawing to show an analysis mesh model;

FIG. 11 is a drawing to show an analysis mesh model zoomed in at thepart near the liquid surface; and

FIG. 12 is a drawing to show the difference in generated stressesobtained as the result of the analysis.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

Explanation will be given below of an embodiment of the presentinvention, referring to the drawings.

FIG. 1 shows an embodiment example of a thermal load reducing system forreduction of stress near the liquid surface of a reactor vessel.

Outside the wall of the reactor vessel 1, a guard vessel 2 is providedto catch the coolant in the unlikely event of coolant leakage. In anannulus space 3 of about 150 mm width between the reactor vessel walland the guard vessel, inert gas is filled for the protection of thereactor vessel. On the outer wall of the guard vessel, heat insulatingmaterial 8 is provided to keep the concrete temperature not going up.The reactor vessel inner wall above the coolant surface 9, covered withheat insulating material 10, is heat insulated from high temperature ofthe coolant and this part of the reactor wall is kept at lowtemperature. Therefore, between high temperature reactor wall below thecoolant surface and low temperature reactor wall above the coolantsurface, a temperature distribution in the vertical direction takesplace, causing a thermal stress.

In the present embodiment, outside the reactor vessel wall and in theregion above and below the coolant surface, a heat conductive member,comprised of a good heat conductive material such as graphite etc.,stable for a long time, is positioned. In this example, the presentembodiment is characterized in that the heat conductive plate 20 as theheat conductive member, is positioned without contact, to expedite thethermal conduction in the vertical direction of the reactor vessel walland to reduce the thermal stress. The heat conductive plate 20 as shownin the drawing is supported by structures outside reactor vessel such asthe guard vessel. For example, the dimensions of the heat conductiveplate should approximately be 30 mm in thickness, 1500 mm in length, andpositioned at the distance of about 60 mm measured from the reactorvessel wall to the surface of the plate, vertically positioned longerbelow the liquid level than above the liquid level, for example about500 mm long above the liquid level and about 1000 mm long below theliquid level.

FIG. 2 is a drawing to explain the principle of thermal load reductionnear the coolant liquid surface of the reactor vessel; FIG. 2 (a) forthe case without the heat conductive plate and FIG. 2 (b) for the casewith the heat conductive plate. As mentioned previously, the reactorvessel in a fast breeder reactor is supported by concrete structure, theupper end of which must be maintained at the temperature of 100° C. orlower. During the starting operation, the temperature of the containedcoolant rises from 200° C. to 550° C. A local temperature gradient inthe vertical direction developed during this process, generates a highthermal stress on the reactor wall. Specifically, in case temperaturedistribution during the starting of the reactor vessel is left freely asit goes (FIG. 2 (a)), between the part contacting high temperaturecoolant and the part contacting low temperature gas, there appears asteep temperature gradient i.e. a bending edge of the temperature curve,at the time when the temperature rise ceased. Accordingly, a maximumstress appears on the outer surface of the reactor wall (the point S inFIG. 2 (a)) near the liquid surface at the time when temperature riseceased. To moderate this problem, as shown in FIG. 2 (b), the heatconductive plate is heated up with the heat radiation from the hightemperature reactor wall below the liquid level, and in turn, the lowtemperature reactor wall above the liquid level is heated up with theheat radiation from the heat conductive plate. This makes it possible toreduce the temperature gradient in the vertical direction, which causesthe stress. As the result, the temperature gradient at point S where themaximum stress taking place, is smoothed at the time T when the maximumstress being generated, and the thermal load is reduced. This embodimentis characterized not only in low-cost since nothing more than a simpleheat conductive plate being added, but also in sure operation since theplate is a non-contacting and static structure.

FIG. 3 is a drawing to show the relationship between equivalent heattransfer coefficient due to radiation and temperature in use.

The thermal radiation/conduction quantity between the parallel planes isexpressed in the following Formula 1. $\begin{matrix}\begin{matrix}{\quad{q = {{ɛ_{eq}\sigma\quad( {T_{1}^{4} - T_{2}^{4}} )} = {ɛ_{eq}\sigma\quad( {T_{1}^{3} + {T_{1}^{2}T_{3}} + {T_{1}T_{3}^{2}} + T_{2}^{3}} )( {T_{1} - T_{2}} )}}}} \\{= {h_{eq}( {T_{1} - T_{2}} )}} \\{ɛ_{eq} = \frac{1}{\frac{1}{ɛ_{1}} + \frac{1}{ɛ_{2}} - 1}}\end{matrix} & \lbrack {{Formula}\quad 1} \rbrack\end{matrix}$where, εl and ε2 is emission rate dependent on materials (quantity ratioin heat received versus heat radiated), between reactor wall made ofstainless steal and high chrome steel e.g. 12% Cr steel, a value morethan 0.1, in case of graphite a value more than 0.8 and σ isStephan-Boltzmann constant. Supposing the reactor wall of stainlesssteel and heat conductive plate of graphite, assigning the minimumvalues of emission rate estimating the effect of heat conductive plateat conservative side, εl=0.1 and ε2=0.8 into Formula 1, calculating therelation between equivalent heat transfer coefficient heq and T(°C.)=T1=T2 (thermally equal condition), FIG. 3. will be obtained. In FIG.3, the horizontal axis indicates temperature (T) of the reactor wall orof the heat conductive plate (graphite), and the vertical axis indicatesequivalent heat transfer coefficient (heq). As the temperature of thereactor wall and of the heat conductive plate going up, the heattransfer coefficient sharply increases. And at around 600° C., theradiation heating seems to perform a heat transfer close to forcedconvection gas heat transfer (approx. 5 W/m² K).

In order to verify the effect of the reduction of thermal load, near thereactor vessel liquid level by means of a heat conductive plate, in bothcases of without and with a heat conductive plate, the maximum stressesgenerated at the start of the reactor operation, were calculated throughnumerical experiment using finite element method and compared.

Here, cases using graphite or 12% Cr steel for heat conductive materialwere analyzed. However, since graphite greatly differs in its heattransfer coefficient due to the difference in crystal structures,conservatively lower heat transfer coefficient was used, making theeffect of heat conductive plate underestimated. Table 1 shows physicalvalues of reactor wall made of stainless steel. TABLE 1 Transient HeatThermal Transfer Specific Coefficient Expansion Coefficient HeatTemperature of Young's Poisson's C efficient Density (xE − 6 kcal/mm s(kcal/kg? (° C.) (N/mm**2) Ratio (10E−6 mm/mm/° C.) (xE + 3 kg/m**3) °C.) ° C.) 20 1.54E+05 0.3 15.15 7.97 3.5556 0.114 50 1.54E+05 0.3 15.657.97 3.6389 0.117 75 1.54E+05 0.3 16.07 7.97 3.7600 0.118 100 1.54E+050.3 16.48 7.97 3.8611 0.120 125 1.54E+05 0.3 16.86 7.97 3.9722 0.122 1501.54E+05 0.3 17.22 7.97 4.0556 0.123 175 1.54E+05 0.3 17.55 7.97 4.16670.125 200 1.54E+05 0.3 17.85 7.97 4.2778 0.127 225 1.54E+05 0.3 18.127.97 4.3611 0.127 250 1.54E+05 0.3 18.36 7.97 4.4722 0.128 275 1.54E+050.3 18.58 7.97 4.5556 0.129 300 1.54E+05 0.3 18.79 7.97 4.6111 0.130 3251.54E+05 0.3 18.99 7.97 4.7222 0.130 350 1.54E+05 0.3 19.19 7.97 4.80560.132 375 1.54E+05 0.3 19.39 7.97 4.8888 0.133 400 1.54E+05 0.3 19.577.97 4.9722 0.133 425 1.54E+05 0.3 19.75 7.97 5.0278 0.133 450 1.54E+050.3 19.93 7.97 5.1389 0.134 475 1.54E+05 0.3 20.11 7.97 5.2222 0.136 5001.54E+05 0.3 20.28 7.97 5.3056 0.136 525 1.54E+05 0.3 20.45 7.97 5.38690.137 550 1.54E+05 0.3 20.80 7.97 5.5000 0.137 575 1.54E+05 0.3 20.747.97 5.5556 0.138 600 1.54E+05 0.3 20.87 7.97 5.6389 0.138

In addition, Table 2 shows physical values of a heat conductive platemade of graphite, and Table 3 shows the same but made of 12% Cr steel,respectively. TABLE 2 Specific Heat Transfer Density Heat Coefficient(×E + 3 kg/m**3) (kcal/kg?° C.) (kcal/mm s ° C.) 2.25 0.165 0.0033 + 00

TABLE 3 Specific Heat Heat Transfer Temperature Density (kcal/kgCoefficient (° C.) (×E + 3 kg/m**3) ° C.) (kcal/mm s ° C.) 0 7.8600.1061 5.422E−06 400 7.860 0.1746 7.452E−06 450 7.860 0.1873 7.643E−06500 7.860 0.1999 7.810E−06 550 7.860 0.2174 8.049E−06 600 7.860 0.23488.264E−06 650 7.860 0.2695 8.192E−06 700 7.860 0.3039 8.096E−06

The emission rates also, were assumed in conservative figures, 0.1 forthe case of reactor wall made of stainless steel of 12% Cr Steel, and0.8 for the case of graphite, were adopted respectively as shown inTable 4. TABLE 4 Stephan- Boltzmann View Emission Constant Factor Rate(kcal/m2hrk4) Reactor Wall (Stainless Steel) 1.0 0.1 4.88E−08 HeatConductive Plate (Graphite) 0.8 Heat Conductive Plate 0.1 (12% Cr Steel)

Simulating the thermal load taking place at the start of the reactor,the analysis was carried out with the internal sodium temperature raisedfrom 200° C. up to 600° C. The heating up speed was at a rate of 15°C./hr from 200° C. up to 400° C., and 20° C./hr from 400° C. up to 600°C. The rise in liquid level to the temperature rise of sodium, had alsobeen taken into consideration. The rise in liquid sodium as against therise in temperature were assumed for 880 mm for the temperature rangefrom 200° C. up to 400° C., and thereafter for 350 mm from 400° C. up to600° C., respectively. For developing an analytic model, a meshgeneration program for finite element analysis Femap v7.1 and foranalytic tool, a general purpose nonlinear structural analysis systemFINAS v14 had been utilized.

FIG. 4 shows the analysis mesh model made out of FIG. 1, and FIG. 5shows the same zoomed in on the heat conductive plate, respectively. Inaddition, the Table 5 shows the list of elements used for the analysis.TABLE 5 Elements used for developing finite element model Heat Heatconductive 8 node tetragon axisymmetric element Transfer (HQAX8)Analysis Heat conductive 3 node axisymmetric element (FCAX3) Radiationlink 6 node tetragon axisymmetric element (RALINK6) Stress 8 nodetetragon axisymmetric element (QAX8) AnalysisElements Used for Developing Finite Element ModelHeat Transfer AnalysisHeat Conductive 8 Node Tetragon Axisymmetric Element (HQAX8)Heat Conductive 3 Node Axisymmetric Element (FCAX3)Radiation Link 6 Node Tetragon Axisymmetric Element (RALINK6)Stress Analysis8 Node Tetragon Axisymmetric Element (QAX8)

FIG. 6 is a drawing to show the difference in generated stressesobtained as the result of the analysis among the 3 patterns of analysis;a case without thermal stress reduction by means of a heat conductiveplate, a case with graphite heat conductive plate and a case with a 12%Cr steel heat conductive plate. On the drawing horizontal axis indicatesgenerated stress Sn (MPa) and vertical axis indicates verticalcoordinate (mm).

As for the analysis result, the stress intensity range (Sn) on thereactor outer wall, being utilized as the strength designing parameter,had been calculated and indicated. The calculation result proves that Snvaries to the difference between emission rate and heat transfercoefficient of the heat conductive plate. The result of an analysisusing graphite which is good in emission, as heat conductive plate,showed the maximum value of Sn reduced from approx 590 MPa to approx 430MPa, or by about 27%. Also, the result using 12% Cr steel as heatconductive plate, the maximum value of Sn was confirmed to have reducedto approx 500 MPa or by about 15%. This verified that a simpleinstallation using a heat conductive plate, significantly reduced thethermal stress taking place on the reactor wall.

Next, an example is explained whereby a reactor vessel thermal stress isreduced by using different material only, without adding any membersanew.

FIG. 7 shows another embodiment example of the thermal load reducingsystem for reduction of stress near the liquid level of a reactorvessel.

Ordinarily, guard vessel is made of the same material with the reactorvessel. In this embodiment, however, the material of the guard vessel 2being altered to better material in heat conduction coefficient thanthat of the reactor vessel. This embodiment is characterized in that theguard vessel 2 become a heat conductive member, and the guard vesselthermally combined by radiation with the reactor wall, expediting thethermal conduction in the vertical direction to the reactor vessel wall,reduces the thermal stress near the liquid level. The composition of thereactor vessel is identical to that of FIG. 1 except that there is noheat conductive plate.

FIG. 8 is a drawing to explain the principle of thermal load reductionnear the coolant liquid surface of a reactor vessel; FIG. 8 (a) for thecase without a guard vessel and FIG. 8 (b) for the case with guardvessel of good heat conductive material.

Like what has been explained over FIG. 2, during the starting operation,contained coolant temperature rises from 200° C. to 550° C., a localtemperature gradient in the vertical direction developing during thisprocess, causes a high thermal stress on the reactor wall (FIG. 8 (a)).Between the part contacting high temperature coolant and the partcontacting low temperature gas, there appears a steep temperaturegradient i.e. a bending edge of the temperature curve, at the time whenthe temperature rise ceased. Accordingly, the highest stress isgenerated on the outer surface of the reactor wall (the point S in FIG.8 (a)) near the liquid surface at the time when temperature rise ceased.To moderate this problem, as shown in FIG. 8 (b), the guard vessel ofgood heat conductive material is heated up with the heat radiation fromthe high temperature reactor wall below the liquid surface, and in turn,the low temperature reactor wall above liquid surface is heated up withthe heat radiation the from the guard vessel of good heat conductivematerial. This makes it possible to reduce the temperature gradient inthe vertical direction, which causes the stress. As the result, thetemperature gradient at point S where the maximum stress taking place,is smoothed at the time T when the maximum stress being generated, andthe thermal load is reduced. This embodiment is characterized in thatthe method not only avoids any effect to the construction cost with nonew members being added, but also assures operation due to itsnon-contacting and static structure.

FIG. 9 is a drawing to show the relationship between equivalent heattransfer coefficient due to radiation and temperature in use, horizontalaxis indicates temperature (T) of reactor wall and guard vessel (of highchrome steel such as 12% Cr steel), while vertical axis indicatesequivalent heat transfer coefficient (heq).

In this embodiment, assuming a conservative value of emission ratebetween reactor vessel wall and guard vessel of 12% Cr steel, andassigning ε1=0.1 and ε2=0.1, the relation between equivalent heattransfer coefficient heq and T(° C.)=T1=T2 (thermally equal condition)was calculated. As the temperature of the reactor wall and of the guardvessel (of high chrome steel such as 12% Cr steel) going up, heattransfer coefficient sharply increases. And at around 600° C., theradiation heating seems to perform a good heat transfer by way of theguard vessel.

For the reactor structure of simple 316FR stainless steel reactor vesselwithout a special measure for thermal stress reduction and a guardvessel, difference in maximum stresses generated during the temperaturerise up to 600° C. at the start of reactor operation was calculated andcompared through numerical experiment, in cases without considering theradiation heating by guard vessel, with a guard vessel of 316FRstainless steel same with the reactor vessel, and with a guard vessel of12% Cr steel, a good heat conductive material.

Physical values of reactor wall used for the analysis are those shown inTable 1, and those of the good heat conductive material used for theguard vessel are those shown in Table 2 (12% Cr Steel). As to theemission rate liable to uncertainty, for εl and ε2, 0.1 and 0.1 wereassigned respectively, to estimate the effect of thermal stressreduction modestly. For load condition, development of analytic modelsand analytic tool are identical to the case of heat conductive plate.

FIG. 10 shows an analysis mesh model made out of FIG. 7, and FIG. 11shows an analysis mesh model zoomed-in at the liquid surface part.

FIG. 12 (corresponding to FIG. 6) is a drawing to show the difference ingenerated stresses obtained as the result of the analysis. Horizontalaxis indicates generated stress Sn (MPa) and vertical axis indicatesvertical coordinate (mm), showing the analysis results in 3 patterns ofwithout considering the radiation heating by guard vessel, with a guardvessel of 12% Cr Steel, and with a guard vessel of 316FR stainlesssteel. The analysis results were indicated, in accordance with verticaldistribution of the stress intensity area (Sn) along the reactor outerwall, which is utilized as the strength evaluation parameter inrespective design. The case of using 12% Cr steel for guard vessel,compared with the case not considering the radiation heating by way ofthe guard vessel, the maximum value of Sn is reduced from approx 590 MPato approx 519 MPa, or by about 12%. Also, the analysis result using316FR stainless steel as guard vessel, the maximum value of Sn wasconfirmed to have reduced to approx 562 MPa or by about 4.7%. Thisverified that a change in guard vessel material from ordinary one togood heat conductive one, achieves a distinguished thermal loadreduction.

As detailed above applying the present invention, thermal load near thecoolant liquid level can be reduced by placing a heat conductive member,outside the reactor vessel in the area above and below the coolantsurface without contacting to the reactor vessel wall.

Particularly, in case placing a heat conductive plate as a heatconductive member, only by an addition of a simple heat conductiveplate, thermal load near the coolant surface can be reduced, not only ata low cost but also assuring the operation since it is a non-contactingand static structure. Equally, in case using guard vessel as heatconductive member, thermal load near the coolant surface can be reduced,without adding any member anew, not affecting the construction cost andassuring the operation since it is a non-contacting and staticstructure.

1. A reactor vessel thermal load reducing system comprising: a reactorvessel having at least one wall and being partially filled with liquidcoolant to an operating coolant liquid surface level between a bottomand a top of said reactor vessel wall, wherein said reactor vessel wallhas the following vertically defined points: Point A at said bottom ofthe wall, Point C at said operating coolant liquid surface level, PointE at said top of the wall, Point B between Point A and Point C, andPoint D between Point C and Point E; and a reactor vessel walltemperature difference minimizing means, said means comprising a heatconductive member as a guard vessel wall located directly adjacent tosaid reactor vessel wall; heat insulation material distributed on theinside of said reactor vessel wall from point D to point C but not frompoint B to point A; heat insulation material distributed on the outsideof said heat conductive member from point B to point D; wherein duringsteady state operation of said reactor, said heat conductive memberreceives a net amount of heat radiation from said reactor vessel wallfrom point B to point C, and said heat conductive member radiates a netamount of heat to said reactor vessel wall from point C to point D. 2.The reactor vessel thermal load reducing system according to claim 1,wherein said heat conductive member has a higher heat conductioncoefficient than said reactor vessel wall.
 3. The reactor vessel thermalload reducing system according to claim 2, wherein said heat conductivemember is 12% Cr steel.
 4. The reactor vessel thermal load reducingsystem according to claim 3, wherein said heat conductive member is 12%Cr steel.